Photolithography is an important process step in semiconductor device fabrication. In photolithography, a circuit design is transferred to a wafer through a pattern imaged onto a photoresist layer deposited on the wafer surface. The wafer then undergoes various etch and deposition processes before a new design is transferred to the wafer surface. This cyclical process continues, building up multiple layers of the semiconductor device.
The minimum feature that may be printed using photolithography is determined by the resolution limit W, which is defined by the Rayleigh equation as:
                    W        =                                            k              1                        ⁢            λ                    NA                                    (        1        )            where k1 is the resolution factor, λ is the wavelength of the exposing radiation and NA is the numerical aperture. In lithographic processes used in the manufacture of semiconductor devices, it is therefore advantageous to use radiation of very short wavelength in order to improve optical resolution so that very small features in the device may be accurately reproduced. Monochromatic visible light of various wavelengths have been used, and more recently radiation in the deep ultra violet (DUV) range has been used, including radiation at 193 nm as generated using an ArF excimer laser.
The value of NA is determined by the acceptance angle (a) of the lens and the index of refraction (n) of the medium surrounding the lens, and is given by the equation:NA=n sin α  (2)
For clean dry air (CDA), the value of n is 1, and so the physical limit to NA for a lithographic technique using CDA as a medium between the lens and the wafer is 1, with the practical limit being currently around 0.9.
Immersion photolithography is a known technique for improving optical resolution by increasing the value of NA. With reference to FIG. 1, in this technique a liquid 10 having a refractive index n>1 is placed between the lower surface of the objective lens 12 of a projection device 14 and the upper surface of a wafer 16 located on a moveable wafer stage 18. The liquid placed between lens 12 and wafer 16 should, ideally, have a low optical absorption at 193 nm, be compatible with the lens material and the photoresist deposited on the wafer surface, and have good uniformity. These criteria are met by ultra-pure, degassed water, which has a refractive index n≈1.44. The increased value of n, in comparison to a technique where the medium between lens and wafer is CDA, increases the value of NA, which in turn decreases the resolution limit W, enabling smaller features to be reproduced.
Due to outgassing from the photoresist layer and the generation of particulates during photolithography, it is desirable to maintain a steady flow of water between the lens 12 and the wafer 16. For example, as described in US 2004/0075895 the lens and wafer could be immersed in a bath of water supported by the wafer stage, with a pump used to recirculate the water within the bath. However, due to the weight of the water bath acting on the wafer stage, this technique is generally considered undesirable.
An alternative technique, as shown in FIG. 1, is to use a nozzle or showerhead device 20 connected to a water source and a vacuum system, shown generally at 22, to produce a localized stream of ultra-pure, degassed water between the lens 12 and the wafer 16. To prevent the ingress of water into other parts of the tool, for example, the mechanism used to move the wafer stage 18, one or more differential air seals 24 are used. As a result, the vacuum system 22 extracts from the tool a multi-phase mixture of water and CDA. However, the extraction of such a multi-phase mixture from the tool using a single vacuum pump, especially in slug or churn regime flows, can generate undesirable pressure and flow fluctuations upstream of the pump, which could be transmitted back to the tool. This could lead to errors in the photolithography process, for example, through variations in the refractive index of the medium located between the lens and the wafer, or through the transmission of mechanical vibrations to the tool.
It is an object of the present invention to provide a vacuum system for extracting a stream of a multi-phase fluid from a photolithography tool and which can minimize any pressure fluctuations imparted thereby to fluid within the tool.